This article aims to present the application of the Six Sigma method to improve the Energy Efficiency of trains operating on Vale's Carajás Railroad in Brazil, one of the largest corridors for the transportation of iron ore and general cargo, consuming over 300 million liters of diesel annually. One of Vale's commitments is to reduce its greenhouse gas emissions, and currently, it faces the challenge of reversing the trend of increasing diesel consumption due to a projected 2.1% increase in Energy Efficiency. The problem was identified, and the goal was defined based on historical data. Processes were then mapped using a tree diagram, Pareto analysis, sequential graph, boxplot, and histogram. Diesel consumption in major flows was identified, and the causes were investigated and organized in a cause-and-effect diagram, prioritized for statistical investigation through correlation analysis and boxplot. Subsequently, proven causes were prioritized, a series of pilot tests were conducted, and an effective action plan was implemented. The project brought sustainability and cost gains to EFC, reducing diesel consumption by 535,366.73 liters, resulting in a real savings of US$ 546,311.15 and a reduction in emissions of 1,527,500.09 kg CO² eq. The projected real and potential gains for a one-year period amount to US$ 3,520,707.62 in Vale's costs. The project established a new record for the Energy Efficiency of iron ore trains on the EFC.